The field of the invention is the distribution of catalyst in a catalyst regenerator vessel.
Fluid catalytic cracking (FCC) is a hydrocarbon conversion process accomplished by contacting hydrocarbons in a fluidized reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of substantial added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds substantial amounts of highly carbonaceous material referred to as coke is deposited on the catalyst. A high temperature regeneration operation within a regenerator zone combusts coke from the catalyst. Coke-containing catalyst, referred to herein as coked catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone.
A common objective of these configurations is maximizing product yield from the reactor while minimizing operating and equipment costs. Optimization of feedstock conversion ordinarily requires essentially complete removal of coke from the catalyst. This essentially complete removal of coke from catalyst is often referred to as complete regeneration. Complete regeneration produces a catalyst having less than 0.1 and preferably less than 0.05 wt-% coke. In order to obtain complete regeneration, the catalyst has to be in contact with oxygen for sufficient residence time to permit thorough combustion.
Conventional regenerators typically include a vessel having a coked catalyst inlet, a regenerated catalyst outlet and a combustion gas distributor for supplying air or other oxygen containing gas to the bed of catalyst that resides in the vessel. Cyclone separators remove catalyst entrained in the flue gas before the gas exits the regenerator vessel.
There are several types of catalyst regenerators in use today. The conventional bubbling bed regenerator typically has just one chamber in which air is bubbled through a dense catalyst bed. Coked catalyst is added and regenerated catalyst is withdrawn from the same dense catalyst bed. Relatively little catalyst is entrained in the combustion gas exiting the dense bed. Two-stage bubbling beds have two chambers. Coked catalyst is added to a dense bed in a first chamber and is partially regenerated with air. The partially regenerated catalyst is transported to a dense bed in a second chamber and completely regenerated with air. The completely regenerated catalyst is withdrawn from the second chamber.
Complete catalyst regeneration can be performed in a dilute phase fast fluidized combustion regenerator. Coked catalyst is added to a lower chamber and is transported upwardly by air under fast fluidized flow conditions while completely regenerating the catalyst. The regenerated catalyst is separated from the flue gas by a primary separator upon entering into an upper chamber from which regenerated catalyst and flue gas is removed. U.S. Pat. No. 4,197,189 and U.S. Pat. No. 4,336,160 teach a riser combustion zone in which fast fluidized flow conditions are maintained to effect complete combustion without the need for the additional combustion in the catalyst bed collected from the top of the riser.
Oxides of nitrogen (NOX) are usually present in regenerator flue gases but should be minimized because of environmental concerns. Production of NOX is undesirable because it reacts with volatile organic chemicals and sunlight to form ozone. Regulated NOX emissions generally include nitric oxide (NO) and nitrogen dioxide (NO2), but the FCC process can also produce N2O. In an FCC regenerator, NOX is produced almost entirely by oxidation of nitrogen compounds originating in the FCC feedstock and accumulating in the coked catalyst. At FCC regenerator operating conditions, there is negligible NOX production associated with oxidation of N2 from the combustion air. Low excess air in the regenerator is often used by refiners to keep NOX emissions low.
After burn is a phenomenon that occurs when hot flue gas that has been separated from regenerated catalyst contains carbon monoxide that combusts to carbon dioxide. The catalyst that serves as a heat sink no longer can absorb the heat thus subjecting surrounding equipment to higher temperatures and perhaps creating an atmosphere conducive to the generation of nitrous oxides. Incomplete combustion to carbon dioxide can result from poor fluidization or aeration of the coked catalyst in the regenerator vessel or poor distribution of coked catalyst into the regenerator vessel.
To avoid after burn, many refiners have carbon monoxide promoter (CO promoter) metal such as costly platinum added to the FCC catalyst to promote the complete combustion to carbon dioxide before separation of the flue gas from the catalyst at the low excess oxygen required to control NOX at low levels. While low excess oxygen reduces NOX, the simultaneous use of CO promoter often needed for after burn control can more than offset the advantage of low excess oxygen. The CO promoter decreases CO emissions but increases NOX emissions in the regenerator flue gas.
On the other hand, many refiners use high levels of CO promoter and high levels of excess oxygen to accelerate combustion and reduce afterburning in the regenerator, especially when operating at high throughputs. These practices may increase NOX by up to 10-fold from the 10-30 ppm possible when no platinum CO promoter is used and excess O2 is controlled below 0.5 vol-%.
Improved methods are sought for preventing after burn and generation of nitrous oxides. Thorough mixing of catalyst and combustion gas in a regenerator promotes more uniform temperatures and catalyst activity fostering more efficient combustion of coke from catalyst.